ENERGETICS

ΔG = ΔH - TΔS

As a manufacturer….. What is important in Chemistry?

• Thermodynamics (Energetics) tells you whether a reaction is possible

• Kinetics (Rates of reaction) tells you how long it takes

• Equilibrium tells you the extent of the reaction

15.3 Entropy (1.5h)

15.3.1 State and explain the factors that increase the entropy of a system

15.3.2 Predict whether the entropy change (S) for a given reaction or process is positive or negative.15.3.3 Calculate the standard entropy change for

a reaction (S) using standard entropy values ( SΘ )

15.4 Spontaneity (2.5h)

15.4.1 Predict whether a reaction or process will be spontaneous by using the sign of ∆GΘ.

15.4.2 Calculate ∆GΘ for a reaction using the equation ∆GΘ = ∆HΘ T∆SΘ or by using values of the standard free energy change of formation, ∆Gf

Θ

15.4.3 Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the

equation ∆GΘ = ∆HΘ T∆SΘ.

Introduction:•Thermodynamics is the study of heat energy changes and other related quantities which occur during physical and chemical changes.•A spontaneous change, once begun, continues on its own with no further outside assistance.•Thermodynamics is able to tell us whether a change is spontaneous or not - if we consider all of the possible factors which affect spontaneity.•The heat energy content change is called the enthalpy change of a reaction.•Exothermic reactions, where ΔH is negative, occur more frequently than endothermic ones and so appear to be more favoured.•Entropy is a measure of the ways of sharing energy packets between particles and of the way the particles can arrange themselves in space. Entropy is often called a measure of the randomness of a system and it is also a measure of the probability of a particular arrangement occurring.

Spontaneous changes occur when enthalpy and entropy changes

combine to give the products more probability of occurring than the reactants.

What is Entropy ΔS ?Entropy may be defined as : ‘The trend in a chemical reaction is from order to disorder’‘order’ means few arrangements of energy‘disorder’ means many arrangements of energy

For the states of matterSolids (least entropy) < liquids < gases

Units of entropy

• Symbol S• Units JK-1mol-1

• Other thermodynamic units are KJ K-1mol-1

• Entropy increases with temperature• Entropy is a measure of disorder• Simple molecules have lower entropies than

complicated molecules.

2nd Law of Thermodynamics

• In a spontaneous change( reaction) the total entropy change must increase

• Total entropy means that of the system and surroundings.

• Entropy can explain why some reactions occur, even though the enthalpy change is not favourable (ie endothermic reactions)

Consider the following reactions and decide whether entropy increases or decreases.

1. N2(g) + 3H2(g) → 2NH3(g)

2. MgO(s) + 2HCl(g) → MgCl2(s) + H2O(g)

3. CO2(g) + Mg(s) → 2MgO(s) + C(s)

4. SO2(g) + 2H2S(g) → 3S(s) + 2H2O(l)

5. C2H6(g) + 3.5O2(g) → 2CO2(g) + 3H2O(l)

Now your turn determine whether entropy increases or decreases ?

• NH4NO3(s) --> N2O (g) + 2H2O(g)

• 2H2O2(aq) --> 2H2O (g) + O2 (g)

• PH3(g) + HI (g) --> PH4I(s)

• 3O2(g) --> 2O3 (g)

• CO2(g) + C (s) --> 2CO (g)

• Ni(s) + 4CO (g) --> Ni(CO)4 (g)

Calculating an entropy change

Write down the balanced chemical equation for the complete combustion reaction between methane and oxygen

Use you data book to look up the standard entropies of each substance in the balanced chemical equation and list them below.

Try to remember that these entropies are for 1 mole of a substance when you use them to calculate the entropy change for this reaction.

What does the sign of your calculated entropy change tell you about the expected spontaneity of this reaction?

When you carry out this reaction in the lab, is it spontaneous?

Look up the standard enthalpy change of combustion for methane and use it to calculate the standard Gibb’s Free Energy for this reaction using the mathematical equation

After a class discussion, write down how you would calculate the standard Gibb’s free energy change of the combustion of methane using the standard Gibbs’ free energies of formation of the chemicals involved.

ΔG = ΔH - TΔS

Some reactions tend to be spontaneous because they give off energy in the form of heat

Others are spontaneous because they lead to an increase in the disorder of the system)

Calculations of ΔH and ΔS can be used to probe the “driving force” or the most important factor in deciding whether a particular reaction is spontaneous or not

ΔH is exothermic and ΔS is positive, then the reaction will be spontaneous

Other ±combinations of ΔH and ΔS are tricky to deal with.

What happens when one potential driving force is favorable and the other is not?

Gibbs Free Energy

We can answer this question by defining a new quantity known as the Gibbs free energy (G) of the system, which reflects the balance between these driving forces. The change in the Gibbs free energy of the system that occurs during a reaction is equal to the change in the enthalpy of the system minus the change in the product of the temperature times the entropy of the system.

If the data are collected under standard-state conditions, the result is the standard-state free energy of reaction

ΔG = ΔH - TΔSThe beauty of the equation defining the free energy of a system is its ability to determine the relative importance of the enthalpy and entropy terms as driving forces behind a particular reaction.

If ΔH is negative (exothermic) and ΔS is positive then we can guarantee that ΔG is negative and the reaction is spontaneous at any temperature

ΔG = ΔH - TΔS

A REAL QUESTION!

(a) Explain in terms of ΔGΘ, why a reaction for which both ΔHΘ and positive is sometimes spontaneous and sometimes not. [4]

(b) Consider the following reaction.

N 2 (g) 3H 2 (g) → 2NH 3(g)

(i) Using the average bond enthalpy values in Table 10 of the Data Booklet, calculate the standard enthalpy change for this reaction. [3]

(ii) The absolute entropy values, S, at 300 K for are 193, 131 and 192 J K−1 mol−1 respectively for N 2(g), H 2(g) and NH3 (g). Calculate for ΔSΘ the reaction and explain the sign of. ΔS Θ [2]

(iii) Calculate ΔGΘ for the reaction at 300 K. [1]

QUESTION CONTINUED

(iv) If the ammonia were produced as a liquid and not as a gas, state and explain the effect this would have on the value of ΔHΘ for the reaction. [5]

(c) Define the term standard enthalpy of formation, and write the equation for the standard enthalpy of formation of ethanol. [2]

(d) Bond enthalpies are tabulated as average bond enthalpies. Explain what this term means. [4]

(e) Enthalpies of reactions, for example combustion, can be calculated using average bond enthalpies or enthalpies of formation. The two methods give closer results for cyclohexane than they do for benzene. Explain this difference. [1]